Fv-1 restriction in NIH and C3H (Fv-1n) cell lines were studied using tropic pseudotypes of MSV. Evidence for two levels of restriction was found in preliminary experiments. One type of restriction appears to be similar to that previously reported in Balb/3T3 (Fv-1b) cells, while an additional restriction, which may or may not be related to the presence of the Fv-1n gene, may also be present.